Workpieces of titanium, titanium alloys, nickel, nickel alloys and chrome-nickel steel, in particular, always have an oxide layer on their surface; once it is removed by chemical or mechanical means, the oxide layer forms again spontaneously when the workpiece is exposed to air or is immersed in aqueous media.
As a result of oxide layer formation, a firmly bonded metal coating of the work piece is possible only if these oxide layers are removed before the coating operation is commenced. The subsequent coating step is effected in an organic electrolyte medium in which the workpiece is immersed. These operations including removal of the oxide layer, must be performed under absolutely oxygen- and water-vapor-free conditions in closed apparatuses, which are only exposed to argon or nitrogen gases, for example.
In order to remove oxide layers from workpieces made of the above-named metals or alloys, methods to accomplish this, which are performed in a vacuum are known e.g. where cleaning is effected by means of sputtering (German laid-open application DE-OS No. 28 09 444). A cleaning method using metal melts, which are covered with a fluxing medium, is also known (U.S. Pat. No. 2,992,135). In this latter method, especially when diffusion annealing is performed to attain improved adhesion, there may be an undesirable formation of intermetallic phases, which cause the material to become brittle (G. E. Faulkner and W. J. Lewis, "Recent Development in Ti Brazing", DMIL. Mem. (1960) No. 45, Battell Mem. Inst., Columbus, Ohio; and H. R. Ogden and F. L. Holden, "Metallography of Ti Alloys", TML Report 103, Battell Mem. Inst., Columbus, Ohio). This method is also unsuitable for manufacture of expensive, finally finished workpieces, where strict demands involving dimensional accuracy are made, because dimensional accuracy is adversely affected at the high temperatures of melting. The vacuum methods mentioned above are not only susceptible to failure but are also associated with the disadvantage of requiring very high capital investments.
It is known (E. L. White and P. D. Miller, and R. S. Peoples, "Antigalling Coatings and Lubricants of Ti", TML Report 34, Titanium Metallurgical Laboratory, Battell Institute) that a coating of a titanium substrate with aluminum causes a reduction in tool wear in thermoforming processes and also prevents heavy oxidation of the titanium. It is also known that metallic coatings on titanium surfaces improves the adhesion of lubricants thereto and thus counteracts heavy frictional wear thereof (N. Factica, "Lubrication of Ti", WDL Techn. Report 57-61 II ASTIA Doc. 155564 (1958); de F. G. A. Laat and T. Adams, "Inhibiting the Wear and Galling Characteristics of Ti", Metals Eng. Quarterly 8 (39-48) (1968); D. L. Padberg and J. J. Crosby, "Fretting-Resistant Coatings for Ti Alloys", 2nd International Conference Ti 1972, MIT, Cambridge, Mass. and E. P. Kingsbary and E. Rabinowicz, "Friction and Wear of Metals", Trans. ASME, Paper 58, Lub. 6 (1968)). Coating titanium with silver facilitates practice of a simple hard-soldering process (H. R. Ogden and F. L. Holden, "Metallography of Ti Alloys", TML Report 103, Battell Mem. Inst., Columbus, Ohio). It is further known that zinc layers, which are applied to titanium, serve to protect the substrate against contact corrosion in titanium-combination elements, which are inserted into aluminum (Metalworking Production, Zinc-Plated Titanium 104 (No. 30, P. 9, 1960)).
There is accordingly a frequent need for coating workpieces of metals, in particular those made of titanium, titanium alloys, nickel, nickel alloys, and chrome-nickel steels with a metal, particularly aluminum, zinc or silver. Especially when such coating is practiced using electrolytic means and the above-named metals and metal alloy, coating compositions, the interfering oxide layer must be removed prior to coating, while the dimensional accuracy of the workpieces is precisely adhered to.